A Doppler radar motion-sensing system typically transmits a continuous-wave (CW) signal, which is reflected off a target and then demodulated in a receiver. According to Doppler theory, a target characterized by a time varying position, but a net zero velocity, will reflect a transmitted signal after having modulated the phase of the signal proportionately to the position of the time-varying target.
Microwave Doppler radar has been used for wireless sensor applications now for a number of years. Among the more prevalent applications of microwave Doppler radar are weather sensing, position and distance sensing, and automobile speed sensing. More recently, however, microwave Doppler radar has been receiving increased attention as a remote-sensing device for health-related and life-signs monitoring and detection. In the fields of health-care monitoring and life-signs sensing, microwave Doppler radar has been used for sensing physiological phenomena, for sensing signs of life to locate persons trapped in earthquake rubble, and cardiopulmonary monitoring of patients afflicted with sleep apnea syndrome.
For example, consistent with the Doppler theory referred to above, the chest-wall of a person such as a monitored patient can be targeted, and a CW radar-type sensing system will receive reflected from the target a signal similar to a signal transmitted to the target. The phase of the reflected signal, however, will be modulated by the time-varying position of the person's chest-wall. The heartbeat and/or breathing signals of the person can be monitored by phase demodulation, which will thus provide a signal proportional to the chest-wall position and thereby provide information about movement due to the person's heartbeat and respiration.
Detecting and measuring cardiopulmonary activity in a human is called for in a wide variety of situations. Cardiopulmonary measurements are typically necessary in the context of medical diagnosis and treatment of a patient, for example. In many situations, there is a need for on-going monitoring of cardiopulmonary activity. Such is the case, for example, with a seriously or chronically ill patient. Monitoring cardiopulmonary activity is especially important, for example, in the case of patients suffering from heart-related and respiratory disorders, such as sleep apnea syndrome. Monitoring cardiopulmonary activity can also be desirable as part of the care of infants or the elderly.
The use of microwave Doppler radar offers the advantage of remote sensing of cardiopulmonary activity, allowing heartbeat and respiration rates to be monitored without direct patient contact. With microwave Doppler radar, heart and respiration signatures are determined based upon the chest motion of a monitored patient, as described above.
A significant limitation on this use of microwave Doppler radar, however, is that such systems typically employ heavy, bulky, and expensive waveguide components that ordinarily are only practical for specialized applications. One approach for obviating these problems is to combine microwave Doppler radar with radio frequency integrated circuit (RFIC) technology. This combination, however, gives rise to its own set of problems. One problem is that the complementary metal-oxide semiconductor (CMOS) oscillators, which are often employed in such RFIC-based systems, suffer from significantly high phase noise—noise much higher than that of a hybrid oscillator incorporating an off-chip, high-quality inductor.
The high phase noise problem is a significant limitation on CMOS motion-detecting radar systems. Since physiological motion is encoded in a phase modulation of the radio signal, close-in phase noise is a critical parameter. This problem can be dealt with by taking advantage of the range correlation phase noise filter effect so as to mitigate the effects of phase noise.
A remaining problem, though, concerns the frequency range of a RFIC-based microwave Doppler radar system. Conventional CW sensing typically utilizes waves that lie in the low-frequency range of the electromagnetic spectrum. Toward the lower end of the frequency range in which such a system typically operates, there is considerable crowding owing to the many other applications operating at or near such frequencies. For example, the 2.4 GHz ISM band is used for wireless LAN, coreless phones, Bluetooth, and other similar applications. Given the ever increasing number of such applications, it is likely that this problem will only worsen in the future. Thus, the low frequency band tends to be crowded since it is also the band in which many other applications operate. Indeed, many if not most industrial, science, and medical (ISM) equipment are operated at RF frequencies within the 2.4 GHz ISM band, a frequency band in which various types of equipment can be operated without the operators having to acquire a license provided that the devices operated comply with maximum emitted power limits.
Perhaps even more problematic is the fact that the low-frequency electromagnetic waves utilized have relatively long wavelengths, making them less sensitive to small displacements of a monitored target. The lessened accuracy is a particular problem with respect to physiological and patient monitoring in which small movements of, for example, a patient's chest wall are the target of the monitoring device.
Accordingly, there is a need, especially for monitoring cardiopulmonary activity, for a system of remote detection that can operate in a frequency band higher than that of potentially interfering applications. Moreover, there is a need for system that can effectively and efficiently perform remote detection while operating in the higher frequency band.